|Bull depicting Ss. Peter and Paul, founders of the Church|
The papal bull had become commonplace by the 13th century. We know they existed as far back as the 6th century, because the lead seal itself exists, even though the message itself does not. We don't have any original bulls from earlier than 819. At that time, they were still being written on fragile papyrus. Once they switched to vellum (calf skin) or parchment aroun the 1th century, the survival rate of documents increased dramatically.
Why was it called a "bull"? The term comes from the Latin verb bullire (to bubble). Bulls were a lump of material, wrapped around a ribbon attached to a document and stamped with official seals/markings, indicating their authenticity. They were originally clay, but lead became more common—and, occasionally, gold: Byzantine emperors liked to issue golden bulls.
In the case of the popes of Rome, one side of the flat leaden bull would bear the image of Saint Peter and Saint Paul. The "SPASPE" seen in te image above stands for Saint PEter and Saint PAul. The other side would bar the name of the issuing pope.
Bulls also have odd names, because they are called after the first few words of the statement, which does not always indicate their content, as I previously explained in the footnote here. Bulls were also not always commands or "new laws." Clearly, the pope had no way to enforce a bull, as when he issued the one about the Templars that was ignored by Edward II of England. Other notable pulls that weren't necessarily embraced: Exsurge Domine (15 June 1520) demanded that Martin Luther retract 41 of his 95 Theses against the Roman Catholic Church, and Sublimis dei (29 May 1537) forbidding the enslavement of indigenous peoples in the Americas.